The Collaborative International Dictionary of English v.0.48:

Atony \At"o*ny\, n. [Gr. ? slackness; 'a priv. + ? tone,
   strength, ? to stretch: cf. F. atonie.] (Med.)
   Want of tone; weakness of the system, or of any organ,
   especially of such as are contractile.
   [1913 Webster]

WordNet (r) 3.0 (2006):

atony
    n 1: lack of normal muscular tension or tonus [syn: atonicity,
         atony, atonia, amyotonia] [ant: tone, tonicity,
         tonus]